1. Field of the Invention
The present invention relates generally to the field of electrochemical cells, and more particularly to an improved method for pasting electrode substrates. Still more specifically, the present invention, in its most preferred embodiment, relates to a two step operation for pasting thin metal foils in which the first pasting step proceeds or follows the application of a modifier such as a binder or an oxidant. In the most preferred embodiment, the first and second steps are performed by spraying the substrate with litharge and subsequently with a solution of persulfate modifier.
2. Description of the Prior Art
It is known to prepare electrochemical cells having improved recharge and discharge characteristics by using thin, non-perforated lead films. Such cells and their method of manufacture are described in U.S. Pat. No. 5,368,961 issued Nov. 29, 1994 to Juergens for "Thin Plate Electrochemical Cell", U.S. Pat. No. 5,198,313 issued Mar. 30, 1993 to Juergens for "Battery End Connector", U.S. Pat. No. 5,047,300 issued Sep. 10, 1991 to Juergens for "Ultra-Thin Plate Electrochemical Cell", and in U.S. Pat. No. 5,045,086 issued Sep. 3, 1991 to Juergens for "Method For Manufacture Of Electrochemical Cell". In these patents, thin non-perforated positive and negative plates, along with thin layers of active material and thin separator layers are described. Initially, the cells are produced with an excess of electrolyte, but through processing a reduce volume of electrolyte is obtained with respect to the absorptive capacity of the separators and the electrode materials.
The cells are preferably produced using films of lead or nickel having a thickness of approximately 0.002 inches. The active material is also thin, e.g. approximately 0.001 to 0.003 inches. The inter-plate spacing is 0.005 or more inches. The active lead materials include PbO and Pb.sub.3 O.sub.4 or leady oxide for the positive plates and PbO for the negative plates. Conventional separator material may be employed, including those made from glass microfibers. Following the preparation of a stack of foils, active materials and separators, the stack is wound into a cylindrical shape and an electrochemical cell resembling other prior cylindrical batteries is produced.
Such cells have achieved commercial success, but certain problems remain in the manufacturing process and in obtaining the full benefits of the cell design. It is known, for example, that when spray guns are used to apply the active material to the foil, sulfate can cause the spray nozzles to clog over time. The result of using lower sulfate contents is a higher plate density than desired and low porosity, both of which can result in incomplete formation and a high self-discharge rate. The aforementioned Juergens patents indicate that the active material may be applied by either high pressure brush or high pressure spray technologies. After application of the active material, the plates are typically subjected to flash drying by infrared heat sources.
In unrelated disclosures relating to more conventional, flat plate lead-acid batteries, such as automobile batteries, it is known that performance can be improved by adding persulfate salts to basic lead sulfate and adding sufficient water to form a paste. The paste may also include lead oxide compounds. The paste described, for example, in U.S. Pat. No. 5,302,476 issued Apr. 12, 1994 to Kao, et al. for "High Performance Positive Electrode For A Lead-Acid Battery" is useful in forming electrodes which include a current collector and an active material in contact therewith. The electrodes, when formed, are capable of yielding high power discharge, and they have high porosity, high surface area and good strength characteristics.
A solution to the pasting problems inherent in the Juergens techniques described in the aforementioned patents would represent a significant advance in that technology.